1. Field of the Invention
The invention relates to connectors for cylindrical or tubular members, and more particularly to connectors for use on pipe and casing used in oil well drilling, or the like.
2. Description of the Prior Art
When a hole is bored into the earth, as for the production of oil and gas, a large diameter pipe known as surface casing is usually installed into the upper section of the borehole. Surface casing stabilizes the walls of the borehole near the surface where they are more apt to cave in. On holes drilled in offshore waters from platforms, or jack-ups, the pipe is extended from the ocean floor to the deck of the drilling structure and is known as a marine riser. In such case, the riser is an extension of the surface casing and serves to prevent entry of sea water into the borehole.
Make up and installation of pipe such as surface casing from floating offshore drilling structures or vessels is complicated by the actions of waves on the vessel. Various motions of the vessel, of which the most critical are roll and pitch, render the make up of screw-type connections very difficult. Not only is it difficult to stab one casing section into another, but it is also difficult to attain proper alignment during make up, which can result in destructive cross-threading. While running casing of any kind is difficult in such hostile environments, it is particularly difficult for larger casing strings, such as twenty inches or larger. This is due not only to the greater mass of such larger casing but also to the decrease in allowable make-up angle for a given pitch thread as the pipe diameter increases.
To overcome these problems, the industry has resorted to use of large pitch (two or three threads per inch), tapered threads to assist in fast make-up in hostile environments without cross threading. Unfortunately, such coarse threads concentrate stresses due to the depth of the threads in relation to the load carrying cross-sectional thickness of the connector. This greatly increases the likelihood of connector failure. Currently used connectors are also subject to failure if the box joint expands, or "bells out," when the joint is under excessive tension and/or bending type loads.
Accordingly, it is an object of the present invention to provide a technique for connecting pipe such as casing, or the like, that is easier to make up under hostile conditions such as those encountered on floating drilling structures.
It is another object of the invention to provide a pipe connector that is less susceptible to stress failure.
It is a further object of the invention to provide a connector that is less likely to "bell out" and thereby cause a connection failure.
It is a further object of the invention to provide threads which facilitate make up which do not disengage due to expansion of the joint.
Other objects and advantages of the invention will become apparent from a reading of the attached claims and description of the preferred embodiments.